T
H
E

E
L
E
M
E
N
T
S
Book II   Proposition 10
Again, since
    the angle EGF is half a right angle,
and
    the angle at F is right, for it is equal to the opposite angle, the angle at C, [I.34]
    the remaining angle FEG is half a right angle; [I.32]
therefore
    the angle EGF is equal to the angle FEG,
so that
    the side EF is also equal to the side FG. [I.6]
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